In recent years, the use of motor vehicles or other mobile structures to illegally transport concealed individuals has become a problem of significant dimension. Aliens attempting to enter the United States or other countries without proper documentation or authority often conceal themselves in vehicles during border crossings and thus avoid detection. Border guards and other government officials charged with the responsibility for preventing such illegal entry have in the past been forced to resort to an unsatisfactory combination of intuition, circumstantial evidence and uncertain legal guidelines in determining which among a large number of vehicles at a border crossing ought to be searched for hidden persons.
In recent years, some prior art devices and systems have been developed which are capable of electronically examining any given vehicle for the purpose of ascertaining whether a person or persons are concealed therein. When a person is present in a mobile or vehicular structure, interaction between the vehicle mass and the person's heartbeat, breathing, muscle reflections and other involuntary body movements generates low level mechanical vibrations, typically having a frequency in the neighborhood of from two to twenty Hertz, throughout the vehicular structure. Devices designed to detect the presence of the concealed person in the vehicle concentrate on sensing these low level vibrations in some discernable manner. Accordingly, highly sensitive geophones or other seismatic-type transducers are placed in contct with the vehicular structure to obtain the electrical analog counterpart of body-induced vibrations.
One type of seismic detection system is disclosed in U.S. Pat. No. 4,096,477 issued to Greer et al on June 20, 1978. This system analyzes a geophone signal output from a frequency perspective. The frequency component of the geophone signal output lying within the frequency range of interest, i.e., the narrow frequency range immediately surrounding four Hertz, is isolated, and zero-crossings of the isolated frequency component can then be monitored to provide an indication of the low level mechanical vibrations, and hence the presence of the person, in the vehicle.
Although frequency-based seismic detection systems provide entirely satisfactory results in a low noise environment, the technique of monitoring zero-crossings in a selected frequency range is highly sensitive to noise and ambient wind conditions. Wind can rock a car and produce low level vibrations with frequency zero-crossings quite similar to those attributable to the presence of a person in the car, thus leading to false indications of the person's presence. Wind-induced vibration can also act to cancel out the body-induced vibrations caused by the presence of a person in the car, thereby falsely masking the presence of the person.
Means for identifying information-bearing signals in the presence of large amplitude noise signals have, of course, been developed for purposes other than the detection of human beings. One such means is illustrated in U.S. Pat. No. 3,617,998, issued to Freedman on Nov. 2, 1971. With the Freedman system, randomly varying target indicating signals from a transducer are detected against a background of widely varying noise by comparing the root mean square value of the power of the target-indicating signal with the noise power. The Freedman system, however, does not relate to a simple, portable detector for onsite usage in the hands of technologically untrained operators, and the need for such a system and method for personnel detection which is capable of furnishing accurate, reliable indications in high noise, high interference environments remains.